Hot rolled dual phase steel strip having features of a cold rolled strip

ABSTRACT

A hot rolled dual phase steel strip with Thickness® 1.0 mm can be used for producing cold pressed and cut pieces, in particular for the car manufacturing industry, by replacing with the same mechanical performances the cold rolled dual phase steel strip usually employed for these purposes. Said hot rolled strip in peritectic steel has a carbon percentage between 0.06 and 0.15% with chemical analysis without any important addition of chromium and phosphorous, with a constant geometrical profile on the whole length and tolerances, particularly with respect to the thickness, which are typical of a cold rolled strip with parallelism &lt;0.05 mm, crown between strips centre and side edges &lt;0.07 mm, while showing a structure with a microcrystalline fineness better than grade 10 of the ASTM 112 standard in a percentage higher than 80% of the whole structure.

The present invention is directed to a hot rolled dual phase steel strip, having features similar to those of a corresponding cold rolled dual phase steel strip.

Low carbon steel strips of the dual phase type (ferrite-martensite) are known, being cold rolled, which have special geometrical, and metallurgical features, as well as relating to planarity and deformability, so as to render the same particularly adapted to the production of pressed or cut pieces requiring very strict tolerances, particularly when designed for the car manufacturing industry with a thickness of more than 1.0 mm.

It is also known that the dual phase steel strip obtained by hot rolling, such as according to the method disclosed in patents EP 0019193, EP 0072867, U.S. Pat. No. 4,790,889 and U.S. Pat. No. 4,561,910, do not show features of quality, particularly relating to their cold workability, that can be considered comparable with those of dual phase steel strips obtained by cold rolling.

A basic feature for this product, especially when it is intended to form structural portions in the car industry field, is in fact the tendency to be cold shaped, as well as a good mechanical resistance being fit to absorb shocks as a consequence of the crash tests recently developed in the car industry. It has been found that these steels must show a microstructure mainly formed of ferrite and, as a slightest portion, of martensite or bainite, i.e. a structure of high hardness that can be obtained by suddenly cooling the steel from an intercritic temperature comprised between 700 and 800° C. This way the residual austenite enriched with carbon is converted into martensite or bainite, giving rise to grains formed of very hard and brittle needle-shaped structures which, when inserted in a much softer ferritic matrix allow cold shaping of pieces, even of complex shape, being present in a very low percentage, never higher than 20% (martensite) and 30% (bainite).

It is also known that this type of steel requires significant additions of chromium and phosphorous, especially the first mentioned element in order to increase the steel capacity of being hardened and to enhance the production of carbides, whereas the second mentioned element is added to make ferrite harder and cause the yield point to raise. Both elements have also the effect of increasing the tensile strength.

As already stated above, these products are generally derivating from cold rolled and continuously annealed strips, while just during the cooling step after annealing the desired dual phase structure is obtained to achieve the above-mentioned features. On the other hand this type of processing, with cold rolling and subsequent annealing, involves rather important burdens as far as the required costs and time are concerned, whereby it is a steadily more and more felt need in this field that of obtaining a hot rolled strip in dual phase steel which is provided with the same mechanical features of the traditional cold rolled steel.

An object of the present invention is therefore that of providing a steel strip of the above-mentioned type which, unlike the other cold rolled dual phase steels being known so far, has the same features and may replace without problems a cold rolled dual phase steel strip, in particular for cold pressed or cut pieces.

Another object of the present invention is that of providing a steel strip that, even without important additions of chromium and phosphorous, is provided with the same qualities as mentioned, which are peculiar of the steels wherein considerable amounts of these two elements are present.

The strip according to the present invention is preferably, although not exclusively, produced by means of in-line plants of the thin-slab type, as disclosed in EP 0415987 in the name of the present applicant and schematically illustrated in FIG. 1 and is characterized, as set forth in claim 1, by a carbon content comprised between 0.06 and 0.15%, manganese between 1.0 and 2.0% with a chemical composition poorer than that of the strip of this type according to the prior art, without important additions of chromium and phosphorous, as well as by a constant geometrical profile along the whole length, with low tolerances relating to the thickness, comparable with those typical of a cold rolled strip.

Further objects, advantages and features of the dual phase steel strip according to the present invention will be clearer from the following description with reference to the annexed drawings in which:

FIG. 1 schematically shows a casting and in-line rolling plant of the thin-slab type, particularly suitable for manufacturing steel strips according to the invention;

FIG. 2 shows a graph representing mechanical features, particularly relating to the cold pressing, of a dual phase steel strip according to the invention when compared with a cold rolled strip of the same thickness; and

FIG. 3 shows a diagram of the variations, graphically obtained by points, of the frequency with which the presence of certain dimensions of the ferritic grain is statistically detected in a number of coils.

As stated in the foregoing, the dual phase steel strip according to the present invention is preferably, although not exclusively, manufactured in thin-slab plants as schematically shown in FIG. 1, where reference is particularly made to the plant being the object of patent EP 0415987. The following processing steps can be distinguish therein, downstream of the continuous casting step: a) liquid core reduction; b) roughing step directly adjoining to the continuous casting; c) heating in an induction furnace; d) keeping temperature in a furnace provided with internal mandrel; e) finishing rolling; f) compact controlled cooling; and g) coiling on a reel. It has been found in fact that the particular working conditions, typical of this plant, give the final product a particularly thin and homogeneous structure with positive consequences on the chemical-physical characteristics of the final product itself.

The features that, as set forth in claim 1, should be shown by the product, i.e. the hot rolled low carbon steel strip with a dual phase structure (formed of either ferrite and martensite or ferrite and bainite), are basically: a thickness ≧1.0 mm with tolerances comprised between ±0.06 mm and ±0.12 mm up to thicknesses ≦8.0 mm, a parallelism <0.05 mm and a structure with grain fineness better than grade 10 of the ASTM E 112 standard.

In the following table there are indicated, for various thicknesses from 1.5 to 8 mm, the corresponding standard tolerances, respectively for the usual hot coils, the cold rolled strips (distinguished between standard and strict tolerances) and the tolerances pertaining to a dual phase strip according to the invention. In the last column there are also indicated the crown or convexity values, i.e. corresponding to the differences between the values of thickness measured centrally and on the side edges of the strip.

Standard Tolerances Tolerance of the strip EN 10031 of the invention EN 10051 Cold Strips Hot Max Thickness Hot Coils Standard Strict Tolerances Crown ≧1.50 +/−0.17 +/−0.11 +/−0.08 +/−0.06 0.03 1.51-2.00 +/−0.17 +/−0.13 +/−0.09 +/−0.07 0.04 2.01-2.50 +/−0.18 +/−0.15 +/−0.11 +/−0.10 0.04 2.51-3.00 +/−0.20 +/−0.17 +/−0.12 +/−0.11 0.05 3.01-4.00 +/−0.22 +/−0.12 0.06 4.01-5.00 +/−0.24 +/−0.12 0.06 5.01-6.00 +/−0.26 +/−0.12 0.07 6.01-8.00 +/−0.29 +/−0.15 0.07

It is easy to see that the tolerances, as detected for the hot rolled steel strip according to the present invention not only correspond on average to less than one half of the tolerances relating to the traditional hot rolled strips, but are even lower than the strict tolerances of the cold strips having the same thickness.

Furthermore with reference to FIG. 3, it can be observed from a microcrystalline analysis of the structure of a steel strip according to the invention that more than 80% of the grains, detected on average at various positions on the strip and statistically for a number of strips, has lower dimensions than those corresponding to grade 10 of the ASTM E112 standard, and consequently a better fineness than that grade.

These features, together with a breaking strain >20%, make this type of hot rolled strip particularly suitable for fine shearing and hole formation by punching, as well as cold stamping of complex shapes. In particular it has been practically proved that with strips according to the invention it has been possible to form bends at right angles and 180° with a radius ≦3 times the strip thickness for thicknesses ≦3.0 mm and ≦5 times the thickness for strips having thickness ≧3.1 mm without giving rise to defects in the region of maximum stress, this confirming the good plasticity of the material. It is clear that these results have been made possible thanks to the fine grain microstructure with homogeneous development of the grain in every direction, or of the polygonal type, with complete separation of the iron carbides from the ferritic grains. Such a structure eliminates any resilient recovery of the material upon shaping, thus allowing to meet in this way very strict tolerances.

Experimental tests of forming capability have been carried out by comparison with cold rolled strips of the same thickness. From these tests it appears, as resulting from FIG. 2, that FLD lines of the Forming Limit Diagram relating to two different steel strips can be overlapped, thus confirming that the strip according to the invention can suitably replace a cold rolled one. The tests of forming capability which have brought to the graphs of FIG. 2 have been carried out on a strip having thickness of 1.0 mm, at room temperature with a mould having diameter of 100 mm and a stamping speed of 1 mm/s.

Homogeneity and fineness of the microcrystalline structure therefore appear to be the reasons of the particular deformability shown by this type of strip.

Finally a typical example of chemical analysis relating to the steel strip according to the invention is reported in the following, while bearing in mind that it is not the case of a binding composition except for the low carbon and manganese content, without important additions of chromium and phosphorous, contrary to the situation in the known dual phase steels: C 0.06-0.15%, Mn 1.0-2.0%, Si ≦0.80%, P ≦0.010%, S ≦0.005%, Cr <0.30%, Ni ≦0.30%, Mo ≦0.03%, Al 0.030÷0.050%.

It should be noted that in the case of the present invention the percentage at which the chromium and phosphorous elements are present can be limited to the values stated, without any necessity of high amounts of these elements being added, although the same good qualities are maintained, thanks to the fact that the temperature of slab, pre-strip and rolled strip never goes below the critical values beyond which the chromium carbides precipitate and phosphorous is separated from the solid solution. 

1-5. (canceled)
 6. A hot rolled, low carbon dual phase steel strip, having: a structure composed of i) ferrite and martensite or ii) ferrite and bainite, a thickness ≧1.0 mm, a crown between a strip centre and a strip side edge lower than 0.07 mm, a composition: C 0.06-0.15%, Mn 1.0-2.0%, Si ≦0.80%, P ≦0.010%, S ≦0.005%, Cr <0.30%, Ni ≦0.30%, Mo ≦0.03%, Al 0.030÷0.050%, balance Fe and impurities, a constant geometrical profile on a length thereof, thickness tolerances between ±0.06 and 0.12 mm for thickness values of up to 8.00 mm, and a homogeneous microcrystalline structure with fineness better than grade 10 of the ASTM E112 standard for more than 80% of its whole structure.
 7. The dual phase steel strip according to claim 6, having a breaking strain coefficient >20%.
 8. The dual phase steel strip according to claim 6, made of peritectic steel in absence of a significant addition of chromium and phosphor.
 9. The dual phase steel strip according to claim 6, having a parallelism of less than 0.05 mm.
 10. A method to manufacture the dual phase steel strip of claim 6, the method comprising: performing a first liquid core reduction downstream of a continuous casting step; performing a roughing step; performing heating in an induction furnace; subsequently keeping temperature in a furnace with an internal mandrel before finishing rolling; performing a compact controlled cooling; and performing a final coiling on a reel. 